For producing the molded article of inorganic powders for producing inorganic sintered bodies (hereinafter referred to as "molded article") by extrusion or injection molding, various binders have so far been used. The fact is, however, that a binder, as selected with a view to the flowability of mixtures of the inorganic powder and binder from the standpoint of molding, is principally used with little attention being paid to an improvement in the high density of molded articles largely affecting the physical properties of products as well as the ease of binder removal on sintering affecting manufacturers. This originates from a condition of molding that, in extrusion and injection moldings, because of the limited discharge capacity of molding machines, markedly larger amounts of binder are used to ensure the flowability of powder required from the standpoints of the bulk density of molded article and sintering.
Molded articles obtained by extrusion or injection molding have an advantage that the inorganic powder is more homogeneously packed in them than in those obtained by press molding, and therefore, sintered bodies having a uniform particle size are very easy to produce. In press molding, however, molded articles having a powder packing density of about 60% based on the theoretical density are obtained relatively easily because the amount of binder used is generally as small as 5% by weight. in extrusion and injection moldings, a binder of amounts as large as 30% by weight is sometimes used, so that only molded articles having a powder packing density of about 40% are obtained. Such molded articles are defective in that sintered bodies obtained by their sintering are low in density as compared with those obtained from press molding. Although extrusion and injection moldings are superior in productivity to press molding and slip casting, it was difficult, because of their difficulty in obtaining high-density sintered bodies, to apply them to the production of functional ceramics such as translucent ceramics, magnetic ceramics, dielectric ceramics, high-strength ceramics and the like in which the function is exhibited only in the form of a sintered body having a density near to the theoretical one.
With molded articles obtained by extrusion or injection molding in which the powder packing density is low, because of their large shrinkage rate on sintering, it is difficult to maintain the dimentional precision of the sintered body obtained. This becomes a problem particularly with ceramics which are difficult in processing after sintering.
In order to improve the foregoing problems encountered in the extrusion or injection molding of inorganic powders, various kinds of binder have been studied. These binders are roughly divided into water-soluble ones and synthetic ones, but the both, because of their defects described below, are not sufficient to solve the foregoing problems.
The first binders, water-soluble ones, include starch, cellulose derivatives, polyvinyl alcohols and the like, and they are widely in use. These water-soluble binders will serve with less amounts of binder converted to solid matter as compared with the synthetic resin ones. However, when these binders are used in a solution containing water as solvent, the amount of the solution is almost equal to the synthetic resin binders and requires about 20% by weight based on the inorganic powder. As compared with the synthetic resin binders, the water-soluble binder solution is low in viscosity so that it easily penetrates between powders to well disperse the powders. However, due to its poor lubricating property, large quantities of water are necesary to give a required flowability. Further, when powders containing the water-soluble binder are extrustion- or injection-molded, there is caused an essential problem. More specifically, because the strength of a molded article is developed with the vaporization of solvent, the molded article immediately after molding is poor in strength and easily changes shape. Accordingly, it is difficult to handle. For this reason, extrusion or injection molding is carried out so that molded articles having a strength of some degree can be obtained by decreasing the amount of water. But this method, since it is based on the sacrifice of the flowability of powder, causes problems such as increase in discharge pressure as well as abrasion of molding machines by inorganic powders and accompanying staining of molded articles. Further, on removing water by drying the molded articles, cracks are easily generated owing to non-uniform drying, and in order to avoid this, a long period of time is necessary for drying. This also leads to a problem that productivity characteristic of extrusion and injection molding is decreased to a large extent.
As the second binders, i.e. synthetic resin ones, polystyrene, polypropylene, polyethylene and the like are used, but they have the following defects. The synthetic resin binders, because of their high melt viscosity, do not easily penetrate into the aggregates of inorganic powders resulting in insufficient dispersibility of powder. Further, they produce only molded articles which are low in powder packing density. Consequently, there is a problem that homogeneous and high-density sintered bodies are not obtained by sintering of such molded articles. The synthetic resin binders, because they are used for molding in a molten state, produce molded articles of high strength by immediate cooling after molding. Because of this, powder flowability on molding can be improved by using large amounts of binder unlike the water-soluble binders. With an increase in the amount, however, there is a problem that the molded article foams easily due to thermal decomposition of gases generated from the binders on sintering. In order to avoid this foaming, a step, generally called a defatting step, for removing the binder by thermal decomposition is applied. However, this requires a longer period of time than drying of the water-soluble binders, thereby resulting in a great reduction in productivity. There is also a method for producing molded articles of high strength by hardening synthetic resin binders other than the foregoing thermoplastic resins, for example thermosetting resins such as phenol resins, urethane resins, etc. on molding. But these binders have a danger that, when the molding is interrupted, hardening takes place in the molding machines making the continuation of molding impossible.
As described above, in either the case of conventional binders for extrusion or injection molding, there are problems such as the rate of powder packing is low and a long period of time is necessary to remove binders. In addition, molded articles which are weak are produced depending upon the kind of binder; and they are contaminated with impurities or become heterogeneous by increase in the required discharge capacity of molding machines and abrasion of the machines. Consequently, the establishment of binder techniques solving these problems is strongly demanded by the industrial world.